This invention relates to the prevention of damage caused by the misfiring of a thyristor when the power supply is interrupted for a short period of time. This situation may occur for many reasons such as a momentary short circuit on the power line, the starting of a large motor with a locked rotor or the action of removing the line plug of the thyristor equipment from a socket where the circuit may be opened and closed during a period of some milliseconds before the final opening. In these instances, because the thyristor timing capacitor cannot immediately discharge through the resistor connected in series, the capacitor will retain most of the charge during the line voltage off period. When power is returned to the equipment, the time normally taken for the capacitor to reach the retained charge is subtracted from the required time and the thyristor fires too early. This type of misfiring is particularly troublesome with low voltage, line operated power supplies and in many instances component destruction is blamed on transient spikes on the power line instead of momentary dips.